PROJECT SUMMARY/ABSTRACT Early childhood caries (ECC) has devastating and long-lasting health consequences. These include higher healthcare costs, pain, infection, lower self-esteem, delayed speech development, poor performance in school, and subsequent poor oral and overall health outcomes as adults. The prevalence of ECC in children of American Indian (AI) families is considerably higher compared to all races in the United States, with ECC occurrence as early as one year of age. ECC and the closely related health disparity early childhood obesity (ECO), are critical for young AI children, especially in the Navajo community that has limited access to nutritious food and clean, safe drinking water. Diet is a crucial contributing factor to ECC and ECO, where high dietary sugar intake and early initiation of sugar-sweetened beverages (SSBs) before six months of age contribute significantly to both conditions in Navajo infants. Frequent sugar consumption drives the selection of cariogenic bacteria and associated metabolic activities within the oral microbiota that ultimately results in caries development. Acquisition and proliferation of cariogenic bacteria before two years of age have been associated with an increased risk of caries. However, the development of the oral microbiome and the impact of the sugar intake reduction on the pattern of microbiota in high-risk Navajo infants remain largely unknown. Recently, an ECO prevention program, called Family Spirit Nurture, was implemented in the Navajo Shiprock community through a randomized controlled trial. The intervention focused on infant feeding practices and SSB intake reduction. The trial enrolled mother-infant dyads and collected longitudinal oral samples from infants aged 3 to 12 months, maternal and child confounding factors, and ECC status at 1 year of age. This trial provided an unparalleled opportunity for the longitudinal investigation of Navajo infants at an early age with two important specific aims: 1) evaluate the developing oral microbiome of Navajo infants in relation to maternal and child confounding factors and the potential impact of the sugar reduction intervention on the oral microbiome profiles and caries outcomes and 2) investigate the metabolome of Navajo infants? oral microbial communities in correlation with ECC outcomes at 12 months of age. We hypothesize that 1) the sugar reduction intervention may influence the oral microbiome composition toward non-cariogenic species and reduce ECC outcome at 12 months of age, 2) there would be developmental changes in the oral microbiome of caries-free Navajo infants and pathogenic shifts in the oral microbiome pattern for those infants who eventually develop caries, 3) Microbial communities of the caries- affected group at 12 months of age may have a higher level of carbohydrate metabolism-related metabolites compared to the caries-free group, and 4) there exists caries-related metabolite signatures of Navajo infants. The goal of this study is to evaluate if the oral microbial and metabolic profiles of Navajo infants could be used to predict ECC and to develop a therapeutic intervention through future research plans.